The present invention pertains to a phase locked loop and particularly to the use of a phase locked loop in a transmitting system. Phase locked loops traditionally are utilized to stabilize the output frequency of a voltage controlled oscillator relative to a fixed and stable reference oscillator. Generally, the voltage controlled oscillator is operating at a substantially different frequency from the reference oscillator and the signals from the two oscillators are altered, by multiplying, dividing, mixing with another frequency, etc., so that the frequencies are equal and the phases of the two equal frequency signals are compared in a phase detector. The output of the phase detector, which may be a simple DC control voltage, may then be utilized to control the frequency of the voltage controlled oscillator. However, in most systems care must be utilized to ensure that the voltage controlled oscillator does not lock on an undesired harmonic frequency of the reference oscillator. Also, when the load on the voltage controlled oscillator changes the voltage controlled oscillator has a tendency to jump out of lock. Circuits have been devised which cause the frequency of the voltage controlled oscillator to sweep through a band of frequencies, including the desired operating frequency, but the sweep voltage must be cancelled when the voltage controlled oscillator attains lock, which generally means that the loop hunts slightly or operates with something less than the optimum characteristics.
The present invention solves these problems by providing a novel integrator, low pass filter circuit having a sweep circuit connected thereto to provide a linear ramp output during unlock conditions in the loop and providing a quadrature phase detector and lock detector to supply an accurate indication of a lock condition in the loop to deactivate the sweep circuit. Further, the output of the voltage controlled oscillator is supplied to power amplifiers and the like through an RF gate which is turned on gradually by a ramp generator only after the loop has attained the lock condition and the power amplifiers and the like have been turned on. Thus, the loop phase detector operates at the center of its transfer characteristic, which results in a constant phase detector gain and maintains the loop operating with optimum characteristics while preventing sudden loads from being applied to the voltage controlled oscillator and causing it to jump out of lock.